The invention relates to an apparatus and method of manufacturing optical waveguides which comprises non-optically measuring the average temperature of a moving optical waveguide as it exits a heated draw furnace that is heated to a draw temperature. In particular, the apparatus comprises a chamber having a plurality of differential thermopiles to generate an output signal that is representative of a maximum amount of radiant energy radiated by the optical waveguide fiber within the chamber. While the invention may be used in manufacturing other types of optical waveguides, it is especially suited for use in manufacturing silica optical waveguides, and will be particularly described in that connection.
An optical waveguide fiber is manufactured by drawing the optical waveguide fiber vertically from a heated optical waveguide preform located within a draw furnace. Because the moving optical waveguide fiber being drawn is at a high temperature of about 1500xc2x0 C. to 1800xc2x0 C., and due to the small diameter (about 125 microns) of the optical waveguide fiber, non-contact temperature measurement is a preferred choice with such a small, moving or inaccessible optical waveguide. One non-contact way of measuring the temperature of an optical waveguide fiber is the use of radiation thermometers.
Temperature measurement with a radiation thermometer is based on the fact that all objects emit radiant energy. Radiant energy is emitted in the form of electromagnetic waves, considered to be a stream of photons traveling at the speed of light. The wavelengths of radiant energy emitted by a hot object range from the ultraviolet, 0.1 micron to the far infrared, 100 microns. However, the majority of the energy radiated by an object between 1500xc2x0 C. and 1800xc2x0 C. is in the near infrared region, 1.0 to 2.0 microns. Radiation thermometers measure the temperature of an object, such as an optical waveguide, by measuring the amount of thermal electromagnetic radiation received from a spot on the object whose temperature is being measured. The intensity and wavelengths of the radiation emitted by an object depends on the emissivity and the temperature of the object. Emissivity is a measure of an object""s ability to emit radiant energy. The emissivity of an object is the ratio of energy emitted while at a particular temperature to that of a perfect emitter or xe2x80x9cblackbodyxe2x80x9d at the same temperature. Since emittance will differ from one object to another, a standard, called a blackbody, is used as a reference for calibrating radiation thermometers and serves as the basis for the laws that define the relationship of the intensity of radiation and wavelength with temperature. A blackbody is an object having a surface that does not reflect or pass radiation. It is considered a perfect emitter because it absorbs all electromagnetic radiation to which it is exposed and re-emits the maximum spectral radiation allowed by Plank""s law. The intensity of radiant energy increases as temperature increases. Thus, such devices are capable of measuring the temperature by measuring the intensity of the radiation that the object emits.
A radiation thermometer consists of optical lenses that collect and focus the radiant energy emitted by an object, and a radiation detector/sensor converts the focused radiant energy into an electrical signal and an indicator provides a readout of the measurement. A disadvantage of radiation thermometers is that they require a prior knowledge of the optical properties of the object being measured and, more specifically, the emissivity, ∈ of the object. Thermal radiation by an object always contains stray radiation emitted by the environment surrounding the object area and reflected by the object""s surface. Hence, to maintain high measurement accuracy of a radiation thermometer precise compensation/adjustment is necessary. For example, a radiation thermometer that is sensitive to energy in the wavelength range from 4.9 xcexcm to 5.5 xcexcm with a spot size or field of view of 1.1 mm does not yield accurate temperature data when attempting to measure the temperature of a 0.125 mm diameter optical fiber for several reasons. First, the accuracy of a radiation thermometer is a function of the emissivity of the fiber within the sensitive wavelength range of the radiation thermometer, namely, 4.9 xcexcm to 5.5 xcexcm. Emissivity is the ratio of the emitted radiation by an object at specific wavelengths and temperature to the emitted radiation from a blackbody at the same wavelengths and temperature and unfortunately emissivity can be temperature and geometry dependent. The object, in this case is an optical waveguide fiber, which is made primarily of silica. Silica is partially transparent to radiation at wavelengths shorter than approximately 8 microns for certain thicknesses. Also, the effect of the cylindrical geometry of the fiber on its emissivity is not well understood. The above mentioned emissivity uncertainties along with the fact that the fiber occupies approximately only 15% of the thermometer""s field of view as well as vibrating in and out of the field of view make any attempt to measure it""s temperature using a radiation thermometer unreliable. One version of a radiation thermometer attempts to overcome the vibrating fiber issue by using a panning mirror that pans an area looking for the optical waveguide fiber and recording the peak temperatures over time. Thus, when the temperature peaks, it is assumed that the optical waveguide fiber is occupying the maximum 15% of the field of view however the effective emissivity of the fiber is not known and therefore cannot be entered into the thermometer. Another disadvantage of radiation thermometers is that they are quite expensive.
In light of the foregoing, it is desirable to provide an apparatus and method of accurately measuring the temperature of a moving optical waveguide. In addition, it is desirable to provide an apparatus and method that minimizes any stray radiation and/or ambient temperature changes from effecting the temperature measurement of the optical waveguide. Further, it is desirable to provide an apparatus that is rugged and capable of withstanding high temperatures, as well as an apparatus that can consistently provide an accurate average temperature measurement and has a fast response time. A further object of the invention is to provide a reliable method of manufacturing silica glass optical waveguides, while reliably monitoring and controlling the temperature of the waveguide during the manufacturing process. Finally, it is desirable to provide an apparatus that is relatively inexpensive to manufacture.
Accordingly, the present invention is directed to an apparatus and method of manufacturing optical waveguides that comprises non-optically measuring the average temperature of a moving optical waveguide fiber as it exits a heated draw furnace using a temperature device or monitor. In particular, the invention provides an apparatus and method of measuring an average temperature of a moving optical waveguide, where the radiant energy emitted by the moving optical waveguide is non-optically processed by the temperature device. The invention provides an optical waveguide temperature monitor and a method of measuring the average temperature of a moving optical waveguide by non-optically detecting the radiant energy emitted by the moving optical waveguide and non-optically measuring the heat flux radiated by the moving optical waveguide within a chamber that is adapted to receive the moving optical waveguide through a central channel. The principal advantage of the present invention is the provision of an arrangement that overcomes the limitations and disadvantages of the described prior arrangements. Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the apparatus particularly pointed out in the written description and claims hereof as well as the appended drawings.
In accordance with one embodiment, the present invention is a temperature device or monitor used for manufacturing an optical waveguide. The temperature device comprises a thermally isolated chamber having a plurality of side walls and a central channel that traverses the chamber from a top wall to a bottom wall, where the chamber is adapted to receive through the central channel the optical waveguide fiber being drawn. The device further comprises a plurality of heat flux sensors, which in a preferred embodiment are differential thermopiles that are secured to the inner surface of the side walls of the chamber. A first surface of each of the heat flux sensors that faces the central channel has a dark absorptive surface, which is exposed to the thermal energy radiated by the optical waveguide fiber, whereas, a second surface of each of the heat flux sensors is thermally isolated from the first surface and is in thermal contact with the side walls of the chamber. The temperature monitor further comprises a cooling system that is in thermal contact with the side walls, preferably, built into the side walls of the chamber. The cooling system is adapted to substantially maintain the reference surface temperature Ts of the side walls of the chamber. Thus, since the second surface of each of the heat flux sensors is in thermal contact with the side walls, the second surface is substantially maintained at the reference surface temperature of Ts by the cooling system. As such, a temperature gradient develops between the first and second surfaces of the heat flux sensors and this temperature gradient is translated into an output/voltage signal that is proportional to the thermal/radiant energy absorbed or the heat flux measured by all of the heat flux sensors. Based on the amount of heat flux absorbed by the heat flux sensors, the average temperature of the optical waveguide fiber can be determined. Also, each of the heat flux sensors is serially interconnected to generate an aggregate output signal that is substantially proportional to a maximum amount of radiant energy absorbed by all of the heat flux sensors within the chamber. Further, each of the heat flux sensors has an electrode that is connected to a measuring device or readout instrument that registers an aggregate output signal generated by all the heat flux sensors. The output signal is proportional to the thermal/radiant energy absorbed by each of the heat flux sensors. In a preferred embodiment, the measuring device is a voltmeter.
In another embodiment, the invention provides an optical waveguide fiber manufacturing device. The manufacturing device comprises of a draw furnace heated to a draw temperature, and an optical waveguide preform positioned within the draw furnace, where the optical waveguide preform is heated to the draw temperature. The manufacturing device further comprises a temperature monitor for non-contact and non-optical measurement of an average temperature Tf of an optical waveguide fiber being drawn from the heated optical waveguide preform. The temperature monitor is in alignment with and downstream from the draw furnace. The temperature monitor (see comprises a thermally isolated chamber having a plurality of side walls and a central channel that traverses the chamber from a top wall to a bottom wall, where the chamber is adapted to receive through the central channel the optical waveguide fiber being drawn from the optical waveguide preform. The chamber has a plurality of heat flux sensors, with each of the heat flux sensors being mounted onto an inner surface of each of the side walls of the chamber. Preferably, each of the heat flux sensors is serially interconnected to generate an aggregate output signal that is substantially proportional to a maximum amount of radiant energy absorbed by all of the heat flux sensors within the chamber. The aggregate output signal is preferably substantially proportional to the average optical waveguide fiber temperature Tf of a length of the optical waveguide fiber within the chamber. Further, the maximum amount of radiant energy absorbed from the optical waveguide fiber within the chamber is substantially proportional to the fourth power of the average temperature Tf of the length of the optical waveguide fiber within the chamber. The temperature monitor further includes a cooling system that is in thermal contact with each of the side walls of the chamber. The cooling system is adapted to substantially maintain a reference surface temperature Ts of each of the side walls of the chamber.
In another aspect, the invention provides a method of manufacturing an optical waveguide fiber, where the method comprises the steps of providing an optical waveguide preform, and heating the optical waveguide preform to a draw temperature, and then drawing an optical waveguide fiber from the heated optical waveguide preform. The method further includes providing a heat flux chamber having an optical waveguide fiber entrance and an optical waveguide fiber exit, and passing the drawn optical waveguide fiber through the entrance and out the exit of the chamber. Finally the method includes the step of non-optically measuring the heat flux radiated by the optical waveguide fiber within the chamber. Preferably, the step of non-optically measuring includes the steps of serially interconnecting an array of heat flux sensors to an inner surface of a plurality of side walls of the heat flux chamber, and providing a cooling system that is in thermal contact with the plurality of side walls of the heat flux chamber, where the cooling system is adapted to substantially maintain a reference surface temperature of each of the heat flux sensors. The method further comprises adjusting the draw temperature based on a measured heat flux of the of optical waveguide fiber within the chamber, where the measured heat flux is proportional to the fourth power of an average temperature of a length of the optical waveguide fiber within the chamber.
In yet another embodiment, the invention provides a method of measuring an average temperature Tf of an optical waveguide fiber that is being drawn from a heated optical waveguide preform in a draw furnace. The method comprises the step of providing a chamber having a plurality of side walls and a central channel that traverses the chamber from a top wall to a bottom wall. The method also includes the steps of serially interconnecting a plurality of differential thermocouple pairs onto a substrate to form a differential thermopile, and securely fixing each of the differential thermopiles to an inner surface of each of the side walls of the chamber. Further, the method includes serially interconnecting each of the differential thermopiles to a readout instrument that indicates the aggregate output signal generated by the differential thermopiles, the aggregate output signal being representative of the maximum amount of radiant energy absorbed by the differential thermopiles within the chamber. The method includes the step of passing the optical waveguide fiber being drawn through a central channel in the chamber. The method further includes the steps of maintaining a reference surface temperature Ts of one surface in each of a plurality of differential thermopiles, where each of the differential thermopiles is fixed to an inner surface of the side chamber with the one surface of each of the differential thermopiles being in thermal contact with the side walls of the chamber. The method further includes generating an aggregate output signal representative of a maximum amount of radiant energy absorbed by the differential thermopiles within the chamber. The method further includes, providing a cooling system that is in thermal contact with the side walls of the chamber, where the cooling system is adapted to substantially maintain a reference surface temperature Ts of the one surface of the differential thermopiles. The method also includes providing within the side walls of the chamber a plurality of channels that are adapted to receive a coolant from an external chiller that maintains the coolant at the temperature Ts. Also, in a preferred embodiment, the method includes a first thermocouple of each of the differential thermocouple pairs from a second thermocouple, with the first thermocouple being exposed to the radiant energy radiating from the optical waveguide fiber and with the second thermocouple being in thermal contact with the inner surface of the side walls of the chamber. In a preferred embodiment, the method includes providing a chamber that is made of aluminum.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrating embodiments of the invention, and together with the description serve to explain the objects, advantages, and principles of the invention.